The present invention relates to semiconductor devices and more specifically relates to a novel structure and process for connecting a semiconductor die to a conductive support.
Semiconductors devices such as MOSFETs, bipolar transistors, thyristors, diodes and the like are manufactured as elements of a larger wafer which permits the simultaneous formation of the necessary junctions and metalizations of the die which are later separated from the wafer. After the die are separated they are usually mounted on respective heat sink supports, typically the central xe2x80x9cpaddlexe2x80x9d portions of a conductive lead frame, and are then housed within a suitable protective housing.
A common prior art technique is to use a soft solder perform to connect the bottom electrode of the die to a low-cost copper lead frame. The assembly is then heated above the melting temperature of the perform and the solder flows to connect the die to the lead frame. At the same time, a solder fillet, which may be 5 mils wide is also formed around the periphery of the die. Thus, the lead frame element must be larger in area than the die to accept this fillet. Consequently, the ultimate package size is also increased.
It is known that ultrasonic bonding, or xe2x80x9cscrubbingxe2x80x9d can be used to connect the die to a lead frame, and that this process minimizes the width of the solder fillet. In the scrub operation, the solder chosen is heated below its melting point and ultrasonic energy, causing the xe2x80x9cscrubbingxe2x80x9d (or friction) causes the ultimate wetting and adhesion of the solder to the die and the lead frame.
When the scrub process is used, the die is provided with a sputtered gold layer, and a gold perform solder is used between the die and lead frame. It is then necessary to heat the gold and the gold-silicon eutectic to about 380xc2x0 C. and then scrub at about 400xc2x0 C. This process is useful in forming short solder fillets (since the solder does not flow readily). However, the higher temperatures needed prevent the use of the conventional copper lead frame, and a more expensive nickel-iron alloy lead frame is necessary. Further, the gold solder is a xe2x80x9chardxe2x80x9d solder which can fail during thermal cycling due to the mismatch in the thermal expansion characteristics of the gold, silicon and nickel-iron lead frame. Thus, the scrub process has been used principally for very small die.
Finally, gold is very undesirable in most semiconductor device fabrication facilities because even very small amounts of gold atoms which may be released into the fabrication area can contaminate the wafers being processed.
The present invention provides a novel solder-on back metal for semiconductor die which is connectable to a conventional copper lead frame by a relatively low temperature scrub process, and which avoids gold in the solder or system. More specifically, the back of the die carries a titanium layer (which can be applied by sputtering) which is then covered by a thin copper layer, and then by an easily scrubbable thick layer of soft solder such as a lead/tin solder.
The soft solder surface of the die is then placed atop a conventional copper lead frame and ultrasonic energy is conventionally applied to cause the xe2x80x9cscrubbingxe2x80x9d action which attaches the die to the lead frame at just below the melting point of the solder (300xc2x0 C.) with only a very small solder fillet.
The novel lower temperature process permits the use of standard copper lead frames and permits the use of a die area almost as large as the lead frame area, thus reducing the package size compared to one using the conventional solder process in which the solder is heated above its melting point and flows readily. Further, the process of the invention is carried out without the use of gold-containing solders.
While the invention is particularly useful for ultrasonic bonding, it should be noted that the solder-on back metal can easily be soldered to any conductor in any other manner.